The present invention relates to a servo system using a position detector for converting an analog amount such as a physical amount and an angular position of a shaft to a digital amount, for example, a pulse generator like an encoder. Particularly, the servo system according to present invention is suitable for use in business machines, household electric appliances, information machines, working machines, measurement machines, industrial robots and so on, in addition to being suitable for use in a magnetic disk drive unit, an optical disk drive unit, a magnetic tape drive unit and so on, the position and speed of which are digitally controlled by a digital computer.
An apparatus described in JP-A-63-92283 is provided with an electronic circuit having characteristics of a speed detector and a motor which are approximated by a first-order low pass filter. This electronic circuit is constructed so as to serve as an observer for the motor, where a speed signal without phase delay is extracted from the observer to form a speed feedback signal and a phase feedback signal for controlling the speed of the motor. Also, JP-A-63-271728 discloses that an observer is constituted within a controller for interpolating a discontinuous position signal outputted from a position detector for converting an analog amount such as a physical amount and an angular position of a shaft to a digital amount, and an estimated signal generated by the observer is used for a positioning control.
The speed detector disclosed in the above-mentioned official gazette detects an approximate speed, not an instantaneous speed, so that a detection error and a delay are increased particularly in a low speed range, which results in degrading the estimation accuracy of the observer.
Also, when an estimated value generated by the observer includes an error, the foregoing prior art does not consider synchronously extracting an actually detected value and the estimated value in an adjusting process of the observer for reducing the error in the estimated value. Specifically, as a result of extracting a discrete detected value (the same meaning as quantization) and a continuous estimated value and comparing them with each other without considering time synchronization of both, a false speed deviation (error) is generated, and the adjustment of the observer becomes insufficient and unstable, whereby a highly accurate condition estimation by the observer is not expected, and accordingly the control of the servo system is insufficient.